EP1899669B1 - Plate heat exchanger with exchanging structure forming several channels in a passage - Google Patents

Description

The present invention relates to a cryogenic separation apparatus comprising at least one plate and fin heat exchanger according to the preamble of claim 1. FR 2 789 165 describes such an apparatus.

There are different types of plate and fin heat exchangers, each adapted to a field of use. In particular, the invention is advantageously applied to a heat exchanger of an air separation unit or H ₂ / CO (hydrogen / carbon monoxide) mixtures by cryogenic distillation.

This exchanger can be a main exchange line of an air separation apparatus, which cools the incoming air by indirect heat exchange with the cold products from the distillation column, a subcooler or a vaporizer / condenser.

The technology commonly used for these exchangers is that of aluminum exchangers with brazed plates and fins, which make it possible to obtain very compact members with a large exchange surface.

These exchangers consist of plates between which are inserted waves or fins, thus forming a stack of so-called "cold" passages and so-called "hot" passages.

Commonly used exchange waves are straight waves, perforated waves, and partial offset or "serrated" waves.

h (mm) :

hauteur de l'onde (de 3 à10 mm).

e (mm) :

épaisseur de l'onde (de 0,2 à 0,6 mm).

n (m^-1 ou pouce^-1) :

nombre d'ondes par unité de longueur (de 177 à 1102 ondes/m).

perf (%) :

taux de perforation (5% pour les ondes perforées).

I_s (mm) :

longueur de décalage (pour les ondes à décalage partiel ou « serrated »).

These waves are characterized by the following parameters:

h (mm):

wave height (from 3 to 10 mm).

e (mm):

wave thickness (0.2 to 0.6 mm).

n (m ^-1 or inch ^-1 ):

number of waves per unit length (from 177 to 1102 waves / m).

perf (%):

perforation rate (5% for perforated waves).

I _s (mm):

offset length (for partial offset or "serrated" waves).

Thus, the hydraulic diameters (Dh) of the waves conventionally used in soldered plate and fin exchangers are between 1 and 6 mm. These exchange waves are currently formed using a press.

Various means make it possible to increase the exchange surface.

The exchange surface that separates two fluids, consists of a so-called "primary" surface corresponding to the flat surface between the two fluids and a so-called "secondary" surface generally consisting of fins perpendicular to the primary surface and forming thus an exchange wave. It is the number of inserted fins (density of the wave) and the height of the fins which create the increase of the exchange surface.

The denser the wave, the larger the exchange surface. However, there is a manufacturing limit or constraints due to the process. The press tool used to manufacture the wave, makes it possible to obtain maximum densities of 1023 to 1102 waves per meter. The density of the selected wave may be smaller when it is preferable to limit the pressure drops. In addition, under certain operating conditions such as bath vaporizers / condensers, safety constraints limit the number of waves per meter to values well below the maximum values that can be manufactured.

The fins have a temperature gradient. Beyond a certain height of fin (wave), the area in the middle of the fin exchange significantly less well. There is therefore an optimum wave height corresponding to an optimum fin coefficient value. The wavelengths commonly used vary from 3 to 10 mm.

It is also possible to increase the exchange coefficient.

The more turbulent the fluid, the better the exchange coefficient. This turbulence can be generated by a modification of the shape of the channels or by the insertion of obstacles generating turbulence (ex: perforated straight wave, partial offset or "serrated", with sinuous generators or "herringbone", with shutters, insertion of mini-fins, windows, ...).

In the case of the vaporization of a fluid, a surface that has a larger number of nucleation sites has a better exchange coefficient. These nucleation sites are micro-cavities of various sizes and shapes (re-entrant cavities) present on the surface or through a porous layer.

In the case of the condensation of a fluid, the thickness of the liquid film deteriorates the exchange coefficient. It is therefore interesting to drain the liquid by the presence of grooves, perforations or reliefs.

Recently there appeared a type of exchangers called micro-exchangers.

These are exchangers with channels of hydraulic diameters smaller than one millimeter. The reduction in the size of the channels makes it possible to develop the heat exchange surface (gain in compactness of the apparatus). The exchange coefficient then becomes practically inversely proportional to the hydraulic diameter.

• c Mini-canaux tels que : 1 mm<Dh<3mm (correspondants aux grandeurs de Dh des ondes actuelles).
• c Mini-canaux tels que : 200µm<Dh<1mm.
• c Micro-canaux tels que : Dh<200µm.

S. Kandlikar in "First International Conference on microchannels and minichannels 2003," Extending the applicability of the flow boiling correlation to low Reynolds number flows in microchannels " proposes the following classification, according to the hydraulic diameter of the channels:

• c Mini-channels such as: 1 mm <Dh <3mm (corresponding to the Dh values of the current wave).
• c Mini-channels such as: 200μm <Dh <1mm.
• c Micro-channels such as: Dh <200μm.

For mini-channels (200μm <Dh <3mm): the flow laws for conventional pipes still apply

For micro-channels (Dh <200μm): Surface effects are of considerable importance and conventional flow laws no longer apply.

EP-A-1008826 describes a plate heat exchanger with at least one of the passages containing closed tube-shaped auxiliary passages, the maximum width of which is greater than 50% of the distance between two adjacent plates.

The quantity of flow exchanged through an exchanger is given by the following relation: $$\mathrm{\phi }=k\times S\times \mathrm{\Delta }T$$

For a given ΔT, the exchangers can only be improved by increasing the exchange coefficient (k) and / or by increasing the exchange surface (S).

• une augmentation des pertes de charge induite par l'augmentation des turbulences.
• une augmentation du coût de fabrication dû à la complexité des géométries.

In the case of brazed plate and fin exchangers, the increase of the exchange surface by a so-called "secondary" surface reaches its limits due to manufacturing and / or process constraints. The increase of the exchange coefficient by the creation of turbulence is interesting but presents two main constraints:

• an increase in pressure losses induced by the increase in turbulence.
• an increase in the manufacturing cost due to the complexity of the geometries.

Thus, the creation of a new waveform can not generate much better exchange coefficient gains compared to existing waves. As regards the creation of nucleation sites and liquid drainage, these two methods only concern a particular type of exchange, namely vaporization or condensation.

It therefore seems difficult to greatly improve the brazed plate and fin exchangers using the same axes of development as those described above.

In addition, the micro-channel type technology is very expensive (micro-machining of the channels) and remains today reserved for exchangers of very small size: it does not concern today the applications, such as the separation of air in which the flow rate and the difference in temperature are important.

The proposed solution aims to increase the exchange surface by incorporating the already existing surfaces (called "primary" and "secondary") a third exchange surface called "tertiary" surface.

• passage d'échange « multi-ondes »
• ondes d'échange « mini-canaux », ondes extrudées
• ondes d'échange « mini-canaux », tubes capillaires

We propose three devices that allow to add a so-called "tertiary" surface to the exchange waves currently used in brazed plate and fin exchangers:

• "multi-wave" exchange passage
• "mini-channel" exchange waves, extruded waves
• "mini-channel" exchange waves, capillary tubes

According to one object of the invention, there is provided an apparatus according to claim 1.

Preferably each channel is in contact with at least three other channels or a plate and two other channels. The plate may be a plate defining a passage or a secondary plate located in the passage.

• la structure est composée d'une pluralité de cylindres ;
• à l'intérieur d'un passage, il y a au moins une plaque secondaire de forme générale plate parallèle aux plaques définissant les passages ;
• la structure est formée d'une superposition d'ondes d'échange, chaque paire d'ondes d'échange adjacentes étant éventuellement séparée par une plaque secondaire ;
• la structure est formée d'un corps unique contenant une pluralité de canaux ;
• un canal a un diamètre hydraulique entre 200 µm et 1 mm ;
• un canal a un diamètre hydraulique inférieur à 200 µm ;
• un passage a une hauteur d'entre 3 et 18 mm ;
• les canaux ont une section circulaire, ovale, carrée, rectangulaire, triangulaire ou en losange.

According to other optional aspects:

• the structure is composed of a plurality of cylinders;
• inside a passage, there is at least one secondary plate of generally flat shape parallel to the plates defining the passages;
• the structure is formed of a superposition of exchange waves, each pair of adjacent exchange waves possibly being separated by a secondary plate;
• the structure is formed of a single body containing a plurality of channels;
• a channel has a hydraulic diameter between 200 μm and 1 mm;
• a channel has a hydraulic diameter of less than 200 μm;
• a passage at a height of between 3 and 18 mm;
• the channels have a circular, oval, square, rectangular, triangular or diamond-shaped section.

According to another object of the invention, there is provided a cryogenic separation apparatus comprising at least one exchanger as described above.

According to another object of the invention, there is provided an air separation apparatus in which a main exchange line and / or a vaporizer-condenser and / or a subcooler is an exchanger as described above. .

• La Figure 2 des dessins annexés représente en perspective, avec des arrachements partiels, un exemple d'un tel échangeur de chaleur, de structure classique, auquel s'applique l'invention.
• Les Figures 3A, 4A et 5A représentent un passage d'échangeur vu dans le sens d'écoulement des fluides selon l'art antérieur et les Figures 3B, 4B, 4C et 5B représentent un passage d'échangeur vu dans le sens d'écoulement des fluides selon l'invention.

The invention will be described in more detail with reference to the drawings in which:

• The Figure 2 The attached drawings show in perspective, with partial detachments, an example of such a heat exchanger, of conventional structure, to which the invention applies.
• The Figures 3A, 4A and 5A represent an exchanger passage seen in the flow direction of the fluids according to the prior art and the Figures 3B, 4B, 4C and 5B represent an exchanger passage seen in the flow direction of the fluids according to the invention.

In the Figure 2 , the heat exchanger 1 shown consists of a stack of parallel rectangular plates 2 all identical, which define between them a plurality of passages for fluids to put in indirect heat exchange relationship. In the example shown, these passages are successively and cyclically passages 3 for a first fluid, 4 for a second fluid and 5 for a third fluid. It will be understood that the invention covers two-fluid exchangers only or any number of fluids.

Each passage 3 to 5 is bordered by closing bars 6 which delimit it leaving free windows 7 input / output of the corresponding fluid. In each passage are disposed wave-waves or corrugated fins 8 serving both thermal fins, spacers between the plates, especially during brazing and to prevent any deformation of the plates during the implementation of fluids under pressure and guiding the flow of fluids.

The stack of plates, closing bars and wave-spacers is generally made of aluminum or aluminum alloy and is assembled in a single operation by soldering in the oven.

Fluid inlet / outlet boxes 9, of generally semi-cylindrical shape, are then welded to the exchanger body thus produced so as to cover the rows of corresponding inlet / outlet windows, and they are connected to conduits 10 for supplying and evacuating fluids.

The channels can be formed using various techniques, as described in Anton GRUSS's "Micro heat exchangers" in Techniques de l'Ingénieur, 06-2002.

The solution of the Figure 3B consists in replacing the exchange wave conventionally used of the Figure 3A by several exchange waves 13 of the same type but of smaller wavelength. These new waves inserted in the same passage of the exchanger are assembled using thin sheets covered with solder 13. These sheets called "tertiary surface sheet" constitute the added surface called "tertiary". In the example there are two sheets separating three waves.

• la hauteur du passage,
• le nombre d'ondes d'échange par passage,
• l'épaisseur de la tôle de la surface tertiaire (à priori égale à l'épaisseur de l'onde),
• la forme de la tôle de la surface tertiaire : pleine ou avec des perforations judicieusement positionnées.

All commercially available wave types can be used by modifying and adapting only the wave height. As a result, all the parameters constituting the geometry of a wave type are adjustable (thickness, density, perforation of the wave, etc.). The other parameters are:

• the height of the passage,
• the number of exchange waves per pass,
• the thickness of the sheet of the tertiary surface (a priori equal to the thickness of the wave),
• the shape of the sheet of the tertiary surface: solid or with perforations judiciously positioned.

For this "multi-wave" technology, the hydraulic diameters are of the order of magnitude of the channel width of a conventional wave (1 / n-e).

n* =

nombre d'ondes sur la hauteur d'un passage (avec des épaisseurs de tôles de surface tertiaire de 0.2mm).

w =

largeur d'un canal.

h canal =

hauteur d'un canal.

Here we give the exchange surface gains for different wave heights and with respect to the classical wave of density n equivalent: Classic configuration * n = 2 h passage (mm) wave (mm) n (m ^-1 ) w (mm) h channel (mm) h wave (mm) h channel (mm) surface gain 5.1 0.2 551.18 1.61 4.9 2.45 2.25 19% 5.1 0.3 393.7 2.26 4.8 2.45 2.15 25% Classic configuration * n = 3 h passage (mm) wave (mm) n (m ^-1 ) w (mm) h channel (mm) h wave (mm) h channel (mm) surface gain 7.13 0.2 944.88 0.86 6.93 2.24 2.04 12% 7.13 0.2 629.92 1.39 6.93 2.24 2.04 24% Classic configuration * n = 4 h passage (mm) wave (mm) n (m ^-1 ) w (mm) h channel (mm) h wave (mm) h channel (mm) surface gain 9.63 0.2 944.88 0.86 9.43 2.26 2.06 13% 9.63 0.2 629.92 1.39 9.43 2.26 2.06 27%

n * =

number of waves on the height of a passage (with tertiary surface plate thicknesses of 0.2mm).

w =

width of a channel.

h channel =

height of a channel.

Here we limit ourselves to channel heights (h channel) of 2 mm minimum (for brazing reasons).

At an equivalent volume, the increase in the number of waves to be stacked in the exchanger causes an increase in the manufacturing cost thereof. The installation cost, however, remains the same.

The solution of the Figure 4B consists in replacing the exchange wave conventionally used of the Figure 4A by a structured wave 17 comprising numerous mini-channels 19 with a square section. This wave can be manufactured by extrusion.

The extrusion manufacturing method makes it possible to imagine any type of channel section shape (rectangular, triangular, round, rhombic, ...).

The Figure 4C shows channels of triangular section.

The main parameters are the height of the passage, the number of channels per passage height, the number of channels per meter of passage width and all the parameters which concern the geometric shape of the channels used (height, width, diameter of the channel ,. ..).

This method of manufacture also allows the possibility of inserting micro or mini fins inside the channels to further increase the exchange surface and / or drain a liquid.

The length of the channels (fluid exchange length) can be divided into several extruded wave modules spaced a few millimeters apart to allow inter-channel communication.

• canaux tels que Dh soit de l'ordre de grandeur de la largeur des canaux dans les ondes classiques (w=1/n-e).
• canaux tels que Dh soit compris entre 200 microns et 1 mm (mini-canaux).
• canaux tels que Dh soit inférieur à 200 microns (micro-canaux).

There are 3 categories of geometry according to the hydraulic diameter of the channels (Dh):

• channels such as Dh is of the order of magnitude of the width of the channels in conventional waves (w = 1 / ne).
• channels such as Dh is between 200 microns and 1 mm (mini-channels).
• channels such as Dh is less than 200 microns (micro-channels).

• Pour les canaux, en dehors de l'invention, tels que Dh soit de l'ordre de grandeur de la largeur des canaux dans les ondes classiques (w=1/n-e), on donne ici les gains en surface d'échange (se) pour différentes hauteurs d'onde et par rapport à une onde classique de même hauteur et de densité n équivalente.

Onde classique h=5.1mm Structure extrudée n (m^-1) w (mm) se (m²/m²) h canal (mm) gain 551,18 1.61 7.18 2.25 19% 393,7 2.26 5.51 2.25 33 % Onde classique h=7.13mm Structure extrudée n (m^-1) w (mm) se (m³/m²) h canal (mm) gain 944,88 0.86 14.73 0.96 40% 629,92 1.39 10.48 1.53 40% Onde classique h=9.63mm Structure extrudée n (m^-1) w (mm) se (m²/m²) h canal (mm) gain 944,88 0.86 19.44 0.97 43% 629,92 1.39 13.63 1.69 42% The exchange surface gains obtained for the 3 categories mentioned above are as follows:

• For the channels, apart from the invention, such that Dh is of the order of magnitude of the width of the channels in the classical waves (w = 1 / ne), the exchange surface gains are given here. ) for different wave heights and with respect to a conventional wave of the same height and density n equivalent.

Classic wave h = 5.1mm Extruded structure n (m ^-1 ) w (mm) se (m ² / m ² ) h channel (mm) gain 551.18 1.61 7.18 2.25 19% 393.7 2.26 5.51 2.25 33% Classic wave h = 7.13mm Extruded structure n (m ^-1 ) w (mm) se (m ³ / m ² ) h channel (mm) gain 944.88 0.86 14.73 0.96 40% 629.92 1.39 10.48 1.53 40% Classic wave h = 9.63mm Extruded structure n (m ^-1 ) w (mm) se (m ² / m ² ) h channel (mm) gain 944.88 0.86 19.44 0.97 43% 629.92 1.39 13.63 1.69 42%

For channels such that Dh is between 200 microns and 1 mm (mini-channels), here we give the exchange surface gains (se) for different wave heights and with respect to a conventional wave of the same height and high density n. Classical wave h = 5.1 mm Extruded structure n (m ^-1 ) se (m ² / m ² ) h channel (mm) gain 1 102.36 12.36 0.2 161% Classic wave h = 7.13mm Extruded structure n (m ^-1 ) se (m ² / m ² ) h channel (mm) gain 1 102.36 16.84 0.2 171% Classic wave h = 9.63mm Extruded structure n (m ^-1 ) se (m ² / m ² ) h channel (mm) gain 1 123.62 20.86 0.2 197%

For the channels such that Dh is less than 200 microns (micro-channels), here we give the exchange surface gains (se) for different wave heights and compared to a conventional wave of the same height and high density not. Classical wave h = 5.1 mm Extruded structure n (m ^-1 ) se (m ² / m ² ) h channel (mm) gain 1 102.36 12.36 0.05 717% Classic wave h = 7.13mm Extruded structure n (m ^-1 ) se (m ² / m ² ) h channel (mm) gain 1 102.36 16.84 0.05 741% Classic wave h = 9.63mm Extruded structure n (m ^-1 ) se (m ² / m ² ) h channel (mm) gain 1 123.62 20.86 0.05 818%

The solution of the Figure 5B consists in replacing the exchange wave conventionally used of the Figure 5A by an adequate number of capillary tubes. The arrangement of the capillary tubes is easily arranged because of their shape. The capillary tubes are covered with solder to ensure the mechanical assembly of the assembly.

The adjustable parameters are the height of the passage, the diameter of the capillary tubes, the thickness of the capillary tubes or the number of capillary tubes per m ² .

We give here the exchange surface gains (se) for different wave heights and with respect to a conventional wave of equivalent density. D _ext is the external diameter of the capillary tube. Classic wave Solution 7 capillaries per pass height h passage (mm) n (m ^-1 ) D _ext (mm) Gain in 5.1 551.18 1.4 50% 5.1 393.7 1.4 96% Classic wave Solution 7 capillaries per pass height h passage (mm) n (m ^-1 ) D _ext (mm) Gain in 7.13 944.88 1.2 23% 7.13 629.72 1.2 73% Classic wave Solution 7 capillaries per pass height h passage (mm) n (m ^-1 ) D _ext (mm) Gain in 9.63 944.88 1.4 10% 9.63 629.72 1.4 57%

In each example, the diameter of the capillary corresponds to the maximum diameter in order to obtain a gain in exchange surface area compared to the conventional solution, a smaller diameter will give a much greater gain in exchange surface area.

Claims (8)

1. Cryogenic separation apparatus comprising at least one brazed-plate heat exchanger, of the type comprising a stack of parallel plates (2) which define a plurality of fluid-circulation passages (3, 4, 5) of a generally flat shape, closure bars which delimit these passages, distributing means for distributing a fluid to each passage of a first series of passages (3, 5) and means for sending another fluid to a second series of passages (4), in which apparatus at least one passage (3) contains at least one organised exchange structure (15, 17, 21) which forms a plurality of channels (19) in the width of the passage, characterised in that each channel (19) is in contact with either at least two other channels or at least one other channel and one plate (2, 13), also forming at least three channels in the height of the passage, and such that a channel has a hydraulic diameter of at most 1 mm and the channels (19) have a circular, oval, square, rectangular or rhomboid cross section.
2. Apparatus according to claim 1, wherein the structure is made up of a plurality of cylinders (21).
3. Apparatus according to either of the preceding claims, comprising, inside a passage (3), at least one secondary plate (13) of a generally flat shape parallel to the plates (2) that define the passages.
4. Apparatus according to claim 1, wherein the structure is formed by superposing exchange fins (15), each pair of adjacent exchange fins optionally being separated by a secondary plate (13).
5. Apparatus according to claim 1, wherein the structure is formed of a single body (17) containing a plurality of channels (19).
6. Apparatus according to any of claims 1 to 5, wherein a channel (19) has a hydraulic diameter of between 200 µm and 1 mm.
7. Apparatus according to any of claims 1 to 5, wherein a channel (19) has a hydraulic diameter of less than 200 µm.
8. Air separation apparatus according to claim 1, wherein a main exchange line and/or a vaporizer-condenser and/or a supercooler is an exchanger according to any of claims 1 to 7.

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